In these years, optical transceivers of approximately 10 Gb/s, such as 10 gigabit small form-factor pluggable (XFP) transceivers are being used in optical transmission apparatuses. For example, in cold districts such as areas around polar regions and in tropical areas around the equator, an optical transmission apparatus can be installed in a building that is not equipped with an air conditioning system or outdoor. In such a case, an optical transceiver whose operation is assured over a wide range of temperatures such as −40° C. to 85° C. is prepared.
In general, the range of temperatures within which the operation of a directly modulated laser (DML) of 10 Gb/s is assured is, for example, −20° C. to 90° C. One of the conditions for expansion of the operating temperature range is to satisfy certain characteristics (the modulation characteristic and the like) of a DML installed in an optical transceiver.
A semiconductor laser module having a temperature regulating function by which the temperature of a DML is kept the same on a thermo-electric cooler (TEC) is known. In addition, a configuration is known in which a heater is provided inside a submount of a semiconductor laser module or outside a semiconductor laser module and the semiconductor laser module is heated when the temperature is low (for example, refer to Japanese Laid-open Patent Publication No. 2001-94200, Japanese Laid-open Patent Publication No. 2005-72197, Japanese Laid-open Patent Publication No. 2002-43671, and U.S. Pat. No. 7,492,798). However, because a semiconductor laser module having a temperature regulating function is expensive, semiconductor laser modules that do not have a temperature regulating function are also used.
The above-described examples of related art, however, have a problem in that the quality of communication of a DML deteriorates at low temperatures. For example, because of a characteristic of a semiconductor laser such as a DML, the ratio of the optical output power to the value of the driving current becomes larger as the temperature decreases. For this reason, if, for example, a DML is used at low temperatures and automatic power control (APC) for causing the optical output power to remain the same is performed, the driving current decreases.